Methods for manufacturing molded parts, which consist in particular of a non-flowable mixture of chip and/or fiber materials and heat-hardenable binders and are pressed in a pressing tool under pressure and supplied heat, are widely known. The Applicant itself has disclosed solutions for this purpose in a plurality of patent applications. Examples include DE 44 305 28 A1 which comprehensively discloses a method and a device for manufacturing molded parts. Furthermore, EP 0956936 A1 discloses, in particular, the design of a multipurpose press for the manufacture of molded parts.
Accordingly, in the prior art, molded parts made of small particles, such as for example wood chips and other cellulose-containing fibers, are manufactured in pressing tools configured accordingly for this purpose. Unfortunately, the achievable strengths of the molded parts are limited by the relatively short length of the particles or wood chips used. This applies, in particular, to the modulus of elasticity. Also, the fracture behavior of molded parts of this type is rather disadvantageous, as the molded parts can break with short fibers and also sharp edges or in a brittle manner. On the other hand, molded parts of this type, which are made of small particles, are particularly advantageous with regard to their uniform and smooth surface. The reason for this is that the molded parts can if necessary also be painted or else structured, for example be provided with pores. Furthermore, differing wall thicknesses can also be produced.
In addition to three-dimensional molded parts made of fine particles, parts of this type in the form of boards are also known. These are, for example, boards comprising crushed wood particles, such as for example chip boards or medium-density fiber boards known as MDF boards for short. In this case too, only limited strength, in particular bending strength and modulus of elasticity, can be achieved. This greatly restricts the use of boards of this type in cases in which stringent demands are placed on strength, or the use thereof as supporting parts.
Therefore, large particles or chips are also used for the manufacture of boards or molded parts. These are, in particular, relatively elongate wood chips, known as strands, such as are described for example in WO 02/074509 A1. The molded parts produced therefrom have relatively high strengths, in particular high moduli of elasticity. They also break resiliently without leaving excessively sharp break edges. Furthermore, these molded parts have relatively low density. Their surface structure is, however, disadvantageous because the large particles or chips are evident on the surface. Direct coating, structuring or painting of molded parts of this type is very difficult or even impossible.
Furthermore, multilayered board-like materials made of plywood or laminated wood or molded parts derived therefrom are also known. In these molded parts, thin veneer layers are coated one above the other and compressed in tools. In the veneer itself the mature wood structure is maintained, thus allowing boards or molded parts having high strength and beneficial fracture behavior to be manufactured. Drawbacks of these boards and parts, however, include the relatively high price of the veneer to be used and the manufacturing process which is difficult to automate. For some years, substitute materials made of oversized wood chips, in particular the above-mentioned strands, have also been used instead of plywood. These are used to produce, in particular, boards known as OSBs (OSB: oriented strand or structural board), i.e. boards containing oriented wood chips. These products have properties comparable to plywood. The process for manufacturing OSBs of this type is in this regard fully automatable.
The known methods therefore allow the manufacture of board-like or three-dimensional parts having high strength and low weight. Markedly deformed parts may also be manufactured, wherein the fracture behavior is more beneficial and, in particular, a high modulus of elasticity can be achieved. Known manufacturing methods using strands preferably utilize light types of wood (poplar, aspen, spruce). The wood is prepared using specific machines to form extensive, oversized wood chips. Subsequently, these chips are dried in continuous dryers to a defined wood moisture content of approximately 1%. Binders (pMDI, aminoplastics) are added in a drum gluing machine. In this case, the chip material must however be treated with care to avoid mechanical damage to the structure of the wood. The glued chips are then supplied to a flow forming machine with which a chip nonwoven is continuously scattered and mold boxes are thus filled. These mold boxes are then transferred manually to the pressing tools, where the chip nonwovens are deposited manually. The molded parts are hardened at a temperature of approx. 170° Celsius and a molding pressure of approx. 3 N/mm2. This produces molded parts which have high strength and beneficial fracture behavior and also low weight.
Drawbacks, however, include the exclusively manual process control which renders large-scale production scarcely possible from a financial perspective. Furthermore, the uniform mold boxes which are used for differing molded part contours give rise to a high proportion of wastage which can be as much as 50% of the material used. Also, differing wall thicknesses cannot be achieved for a uniformly formed nonwoven.
In conclusion, it may therefore be stated that the prior art discloses a large number of different molded parts and boards which can be manufactured with one layer or else with a plurality of layers. However, these parts also have, depending on the composition of the material or of the respective layer, the above-mentioned negative properties. It would be desirable to provide a product which combines as many as possible of the positive properties in the form of a particular combination or of a new composite molded part. An effective and process-optimizable manufacturing method should, in particular, be proposed for this purpose.